This invention relates to a measuring sensor for fluid state determination and a method for measurement using such sensor, and is applicable particularly to the temperature measurement conducted to evaluate viscosity or physical properties of given fluid as well as to the measurement conducted to localize a fluid level.
It should be understood that the term "fluid" used herein includes substance in the form of gas, liquid, solid or mixtures of two or more thereof and the term "state" used herein includes not only stationary or flowing state of given fluid but also a state of this fluid in which composition, phase or temperature thereof is steady or variable with the passage of time.
Various methods for measuring a state of given fluid have already been proposed and the inventors also have disclosed methods for measuring a fluid state using metallic thin wire by hot wire method, for example, in Japanese patent application Disclosure Gazettes No. 1985-152943; 1987-27622; 1987-40246; and 1987-56849. According to these prior arts, changes of physicals properties or the like occurring in given fluid are thermally measured on the basis of change in temperature with the passage of time occurring when the metallic thin wire placed within the fluid is applied with electric current and thereby the state of the fluid is determined.
Of the sensors used in such measurement, the sensor disclosed in Japanese patent application Disclosure Gazette No. 1987-56846 (U.S. Pat. No. 4,762,427) comprises metallic thin wire wound around a holding shaft coated with electrical insulator, this assembly as a whole being coated with electrical insulator, so as to be used with said method.
The method disclosed in Japanese patent application Disclosure Gazette No. 1987-185146 employs, in addition to a measuring sensor, alone, used to measure a fluid temperature, an electrical heating sensor so that the state of the fluid may be determined more accurately on the basis of a differential temperature given by these two sensors.
Usually, means such as resistance thermometer bulb, thermistor, thermocouple and radiation thermometer have been utilized as temperature detecting means for measurement of fluid.
The sensor used in the measuring methods or the like of prior art which have been proposed by the inventors comprises, as mentioned above, the metallic thin wire wound on the shaft and is certainly advantageous in that a length of said metallic thin wire may be dimensioned to be several times of the sensor's own length and thereby an electric resistance thereof may be increased so as to obtain a high heating value from a small current value. Such sensor is advantageous also in that the sensor itself cannot be easily damaged or bent.
However, such prior art is disadvantageous in that when the metallic thin wire is spirally bent while being wound on the shaft a stress/strain is developed in said metallic thin wire and an annealing process is necessary to relieve such stress/strain significantly changes a resistance value with respect to the initial value.
Since it is difficult to presume a degree of this change in the resistance value, it is also difficult to obtain the sensor presenting a desired resistance value. Even when the sensors are mass-produced, there is provided no intersensor compatibility unless the sensors are individually calibrated. The characteristic calibration of individual sensors has been disclosed by the inventors in Japanese patent application Disclosure Gazette No. 1987-51520 (U.S. Pat. No. 4,832,504). However, such characteristic calibration of individual sensors is a time-consuming operation.
To minimize bending of the metallic thin wire, the inventors has proposed an improved sensor in Japanese patent application Disclosure Gazette No. 1989-44838, in accordance with which the metallic thin wire is rectilinealy arranged in the longitudinal direction and bent only at opposite ends so that a stress/strain developed in the metallic thin wire may be substantially reduced. However, this invention requires not only a high technique for manufacturing but also a high precision analyzer such as a computer for high precision measurement.